In controlling a driving force applied to a load by using a servomotor, the output torque of the servomotor is conventionally controlled by torque restriction. Since a motor output, however, is generally applied to the load through a transmission mechanism, the driving force applied actually to the load is controlled only indirectly, according to the prior art method of controlling the motor output. This entails the following awkward situations.
FIG. 2 is a block diagram of a basic circuit of a conventional control circuit for controlling a servomotor using a permanent-magnet synchronous motor. In FIG. 2, symbol E designates a three-phase power source. Reference numeral 3 denotes a rectifier circuit; 4, a transistor inverter; and 1, a transistor PWM control circuit. Also, symbol M designates the permanent-magnet synchronous motor, while numeral 2 denotes a rotor position detector, such as a pulse encoder, for detecting the position of a rotor of the permanent-magnet synchronous motor M.
The transistor PWM control circuit 1 compares a speed command value Vo from a control unit with a present speed Vs of the rotor, which is obtained from a rotor position S detected by the rotor position detector 2. Transistors TA to TF of the transistor inverter 4 are turned on or off to control currents flowing through the U-, V-, and W-phase windings of the permanent-magnet synchronous motor M, thereby controlling the rotating speed of the motor M. If the output torque of the motor M is to be controlled, the transistor PWM control circuit 1 is arranged as shown in FIG. 3.
In FIG. 3, numeral 5 denotes a signal processing circuit; 6 and 7, ROMs; and 8, a differential amplifier. The signal processing circuit 5 delivers a voltage Vs, indicative of the present rotor speed, in accordance with the rotor position detection output S. The ROMs 6 and 7 store a group of U- and W-phase command values to be delivered, so as to correspond to individual rotor positions, in order to make the phase of the resultant current, flowing in the U, V, and W phases, perpendicular to that of the main flux of a magnetic field generated by the rotor. The differential amplifier 8 amplifies the difference between the voltage Vo, indicative of the speed command, and the voltage Vs, indicative of the present speed, from the signal processing circuit 5, and delivers an amplified difference signal. Numeral 9 denotes a filter which has a frequency characteristic such that the gain is lowered at high frequencies, and is increased at low frequencies. Zener diodes ZD1 of the filter 9 serve to clamp the peak voltage. Numerals 50 and 52 designate a D/A converter and a clamping circuit, respectively. The D/A converter 50 serves to convert a torque limiting command PL, as a digital signal, into an analog signal. The command PL, which is supplied from a numerical control unit (not shown) or the like, is used to set the value of the driving force to the load. If an input Vr to an amplifier 51, that is, a voltage Vr corresponding to the difference between the speed command Vo from the filter 9 and the present speed Vs, exceeds a predetermined voltage +Vc or -Vc, which corresponds to a torque limiting command PV, in the form of an analog signal, from the D/A converter 50, the clamping circuit 52 clamps the voltage Vr to the voltage +Vc or -Vc. Numerals 10 and 11 denote multiplying digital-to-analog converters. The converter 10 multiplies a voltage VE, delivered from the amplifier 51, by the U-phase command value delivered from the ROM 6. Likewise, the converter 11 multiplies the voltage VE by the W-phase command value from the ROM 7. Thus, the converters 10 and 11 generate U- and W-phase current commands RTC and TTC, respectively. Numeral 12 denotes an adder for adding the U- and W-phase current commands RTC and TTC and generating a V-phase current command STC, which is shifted from the U and W phases by 120.degree.. Numerals 13 and 14 denote detectors for detecting currents Iu and Iw flowing through the U- and W-phase armature windings of the synchronous motor M. Numeral 15 denotes an adder for adding U- and W-phase currents IR and IT, detected by the U- and W-phase current detectors 13 and 14, to calculate V-phase current IS. Numerals 16, 17 and 18 denote circuits for delivering the current command voltages, which are indicative of the currents to be supplied to the U-, V-, and W-phase armature windings. The circuits 16, 17 and 18 are constructed in the same manner, provided that they are supplied with different input signals. The circuit 16 comprises an operational amplifier 19 for amplifying the difference between the U-phase current command RTC and the present U-phase detection current IR, and a low-pass filter 20 for transmitting only the frequency component of the reference carrier wave, as an output from the operational amplifier 19. The circuit 17 receives the V-phase current command STC and the present current IS, while the circuit 18 receives the W-phase current command TTC and the present current IT. As regards other arrangements, the circuits 17 and 18 are identical with the circuit 16. Numeral 21 denotes a circuit (hereinafter referred to as a PWM signal processing circuit) which is composed of a PWM signal processor and a transistor base drive amplifier. The PWM signal processing circuit 21 compares the signals from the circuits 16, 17 and 18 with the reference carrier wave VA, and generates PWM signals PA to PF for turning on and off the transistors TA to TF of the transistor inverter 4.
With the arrangement described above, the permanent-magnet synchronous motor M is controlled as follows. The difference between the speed command Vo and the present speed Vs, which is generated from the signal processing circuit 5 supplied with the rotor position signal S from the rotor position detector 2, is amplified by the differential amplifier 8, and is delivered as an output voltage Vr through the filter 9. If the voltage Vr is not higher than the clamping voltage +Vc or -Vc set by the clamping circuit 52, it is delivered directly, as an output VE, from the amplifier 51. If the voltage Vr is higher than the clamping voltage +Vc or -Vc, the clamping voltage is delivered as the output voltage VE (=+Vc or -Vc) of the amplifier 51, and is supplied to the multiplying digital-to-analog converters 10 and 11. After receiving an address signal, indicative of the present rotor position, from the signal processing circuit 5, the U- and W-phase ROMs 6 and 7 supply the multiplying digital-to-analog converters 10 and 11 with U- and W-phase command values corresponding to the present rotor position. The multiplying digital-to-analog converters 10 and 11 multiply the error signal VE by the command values from the ROMs 6 and 7, respectively, and generate U- and W-phase current commands RTC and TTC, respectively. The adder 12 adds the U- and W-phase current commands RTC and TTC, thereby delivering the V-phase current command STC. Operational amplifiers 19 in the circuits 16, 17 and 18 amplify the differences between the current commands RTC, STC, and TTC and the present U-, V-, and W-phase current values IR, IS, and It detected by the U- and W-phase current detectors 13 and 14 and the adder 12. The amplified signals are filtered by the filters 20, and voltages corresponding to the individual phase command currents are delivered to the PWM signal processing circuit 21. The circuit 21 compares the voltages with the reference carrier wave VA, and delivers the PWM signals PA to PF to the transistor inverter 4 through the transistor base drive amplifier. Thus, the transistors TA to TF of the transistor inverter are turned on and off to control the speed of the permanent-magnet synchronous motor M.
For example, an injection mechanism of an injection-molding machine may be driven by means of the permanent-magnet synchronous motor M, under the aforementioned speed control, so that resin is injected by means of a screw, and is subjected to pressure maintenance thereafter. In doing this, the pressure to be maintained has conventionally been controlled by controlling the output torque of the motor M, i.e., the driving current of the motor. In this case, if the torque limiting command PL, which is necessary for the pressure maintenance, is delivered from the numerical control unit or other control unit, the command PL is converted into an analog signal by the D/A converter 50, as mentioned before, and the clamping circuit 52 sets the clamping voltages +Vc and -Vc corresponding to the torque limiting command PL. When the injection ends, the screw of the injection mechanism ceases to move, and the motor M nearly stops from rotating, so that the difference between the voltage Vs, indicative of the present speed, and the voltage Vo for the speed command becomes large. As a result, the output voltage Vr from the filter 9 exceeds the set clamping voltage +Vc or -Vc, so that the amplifier 51 delivers the voltage VE corresponding to the set clamping voltage +Vc or -Vc to the multiplying digital-to-analog converters 10 and 11. Consequently, the U- and W-phase current commands RTC and TTC from the multiplying digital-to-analog converters 10 and 11 and the V-phase current command STC from the adder 12 take their respective values corresponding to the set clamping voltage +Vc or -Vc. Thus, the motor M delivers an output torque set in accordance with the torque limiting command PL. The output torque of the motor M can be varied by changing the value of the torque limiting command PL. In the pressure control for the injection-molding machine, therefore, the pressure to be maintained is varied in several steps by multistep setting of the torque limiting command PL.
However, the conventional control method, which is an open-loop method, is based only on the supposition that a driving force corresponding to torque restriction is applied to the load by effecting the torque restriction. More specifically, a torque delivered from the shaft of the servomotor is transmitted through a transmission mechanism or the like, to be supplied as a driving force to a load, e.g., a screw for injection and pressure maintenance. Due to various disturbances, such as friction on the transmission mechanism, etc., or deflection or the like of a spring or ball screw, the force applied actually to the load does not always agree with the value set by torque restriction. In the injection-molding machine driven by the servomotor, if the pressure control is performed by torque restriction, as in the aforementioned case, a difference may often be produced between the set pressure to be applied to the resin and the pressure applied actually to the resin, due to friction on the transmission mechanism or the like.